The invention relates to a protective circuit against overpoling and overloading of electronic consumers connectable via terminal conduits to a power source.
A prior art protective circuit of this kind is described in IBM Technical Disclosure Bulletin Vol. 4, No. 9, February 1962, p. 66. in this circuit, the two power feeding terminals connectable to a power supply are provided with a series-connected fuse and with a zener diode bridging an electronic consumer. Under normal operating conditions, that is when a voltage of a predetermined amplitude and of correct polarity is applied to the terminals of the consumer, the zener diode is in its blocking condition. If the polarity of the power supply is accidentally reversed, or if the applied voltage exceeds or equals the breakdown voltage of the zener diode, then the latter becomes conductive and passes a high current which in turn burns the fuse, thus protecting the electronic device against damage. The disadvantage of this known protective circuit is the fact that, after the removal of the cause of the burning of the fuse, the operability of the entire arrangement is dependent on the availability of a corresponding replacement fuse. Especially in the case when the protective circuit of this kind is employed in a motor vehicle, it frequently happens that a suitable replacement fuse is not at hand.
In another prior art protective circuit for electrical consumers, disclosed for example in the German Pat. No. 2,748,267, a diode is connected in series with the consumer. The diode is oriented in such a manner that, in the case of incorrect polarity applied to the circuit terminals, it is polarized in reverse direction and consequently prevents the energization of the consumer. The disadvantage of this known protective circuit is power loss produced by the diode under normal operational conditions of the device. Due to the power loss, the diode heats up and, in extreme situations such as at very high ambient temperatures, the diode is prone to failure and may cause or contribute to a premature disconnection of an electronic consumer, for example in a motor vehicle. A further disadvantage of this protective circuit results due to the voltage drop across the diode, which reduces the actual voltage applied to the consumer. This voltage drop depends also on the ambient temperature, and for example in the case of extremely low ambient temperature the voltage drop on the diode may cause a malfunction of electrically driven circuits used frequently in motor vehicles for performing controlling functions.
A further prior art protective device is known from the German publication DE-OS No. 2,211,337 which protects against overpoling of a power supply. In this known arrangement, a working contact of a relay is connected in series with a terminal conduit between a power source and the electronic consumer. The coil of the relay is connected in series with a diode, and the series connection bridges the two terminal conduits parallel to the power source. In the case of incorrect polarity of the power source, the diode is biased into its closing direction and prevents a current flow through the coil of the relay, so that its working contact is in open position. The disadvantage of this known arrangement is a relatively slow response of the relay, which in most cases does not provide sufficiently short disconnection of its working contact. Accordingly, it is desirable to provide faster protecting circuits of this kind.